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May 31, 2018
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Add note about predicate expressions #5634

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Add note about predicate expressions
sylveon May 27, 2018
9d2d3df
s/predicate expression/expression filter/g
sylveon May 31, 2018
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14 changes: 12 additions & 2 deletions docs/csharp/language-reference/keywords/try-catch.md
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Original file line number Diff line number Diff line change
Expand Up @@ -87,9 +87,19 @@ catch (InvalidCastException e)
{
// Take some action.
}
}
}
```


> [!NOTE]
> It is also possible to use a predicate expression to get a similar result in a often cleaner fashion. The following example has a similar behavior for callers than the precedent example, the function throws the `InvalidCastException` back to the caller when `e.Data` is null.
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Instead of "predicate expression", I'd prefer the term "exception filter".

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Some additional suggested changes, @sylveon:

  • a often -->an often
  • than the precedent example, --> as the previous example.
  • the function --> The function
  • null --> `null`

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I was following the name used a bit before in the same document.

Using catch arguments is one way to filter for the exceptions you
want to handle. You can also use a predicate expression that further
examines the exception to decide whether to handle it. If the
predicate expression returns false, then the search for a handler continues.

Should I change that too while I'm at it @BillWagner?

@rpetrusha will do.

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thanks @sylveon Yes, let's change it in both locations

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Done and done.

>
> ```csharp
> catch (InvalidCastException e) when (e.Data != null)
> {
> // Take some action.
> }
> ```

From inside a `try` block, initialize only variables that are declared therein. Otherwise, an exception can occur before the execution of the block is completed. For example, in the following code example, the variable `n` is initialized inside the `try` block. An attempt to use this variable outside the `try` block in the `Write(n)` statement will generate a compiler error.

```csharp
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